Structural sandwich panels are often used in aircraft, as floor and/or bulkhead panels. Traditionally a sandwich panel consists of a pair of relatively thin skin sheets, and an intermediate layer of foam or honeycomb material. Bonded together the result is a panel with a high strength-to-weight ratio, which makes it of interest for use in aircraft. Sandwich panels are very resistant to loads that are evenly distributed over a relatively large surface area. However when sandwich panels are subjected to loads where the force is applied to a point of the panel, the intermediate layer can be crushed locally and the skins can delaminate from the core. This characteristic of sandwich panels is a problem when the panels are to be used as floor panels and therefore have to be secured to a panel support structure. The fasteners such as e.g. screws, bolts, nails, often will crush the intermediate layer locally, when passed through the panel in order to secure the panel to the support structure. Therefore it is conventional in the art of securing sandwich panels to support members, to use inserts in the panel. The inserts have a hole through which the fastener may pass in order to attach the panel to the support members e.g. rails or studs. The insert then isolates the forces exerted by the fastener which are necessary to properly attach the panels to the support member. The method of securing the insert into a hole in the sandwich panel is of direct consequence to the amount of shear and compressive forces the panel in connection to the support member is able to withstand.
The inserts used generally comprise a cylindrical body with a vertical cylindrical bore. The shape, size, and form of the vertical cylindrical bore in the body is largely dependant on the fastener which is used to secure the panel to the support members. Bolts, screws, and the like exist in various forms and measurements which will require specifically shaped vertical cylindrical bores in the insert. The shape, size and form of the body part of the insert is largely dependant on the method of securing the insert into the panel. Most inserts also comprise a cylindrical head disc which is larger in diameter than the body portion.
Because of the importance of good connection and stress relief between panel, insert, fastener and support member, various methods for securing inserts in sandwich panels have been developed and are known in the prior art.
U.S. Pat. Nos. 4,242,158 and 4,305,540 to Olson, disclose a method of securing an insert in a sandwich panel using an adhesive for one part of the insert and an anvil for forming the other part of the insert. The insert used in this method comprises a cylindrical head disc as top end of a cylindrical body. The length of the body portion is greater than the thickness of the panel. The insert is introduced in a hole in the panel with the cylindrical body, the head disc of the insert rests on top of the top skin of the panel. The bottom part of the cylindrical body of the insert protrudes below the bottom skin of the panel. The head disc of the insert is secured to the top skin of this panel using a suitable adhesive. The anvil presses against the bottom part of insert and this applied pressure causes this protruding portion to flare over and form a flange which secures the insert to the bottom skin. At the same time compressed air blows excess adhesive from the contacted surface from the insert.
Sandwich panels with thermoplastic skin sheets allow the use of thermoplastic inserts. Until now the securing of thermoplastic inserts into the sandwich panel did not satisfy the structural requirements imposed by aircraft manufacturers. This is also stated by Worthing in his U.S. Pat. No. 4,817,264, column 2, line 27-45. Worthing consequently chooses to use a two part insert. This two part insert consists of a metal flange member and a thermoplastic body portion. The method described in this patent uses an adhesive to secure the head disc to the top skin of the panel and ultrasonic energy to soften the protruding end of the insert and pressure to cause this portion to flare over and form a flange. This flange forms a mechanic connection and is similar to the flange described in U.S. Pat. Nos. 4,242,258 and 4,305,540 to Olson.
The above-mentioned methods for securing inserts in sandwich panels are laborious and time consuming. Specifically, the time necessary to allow the adhesive to sufficiently harden, largely influence the production time and costs of such sandwich panels. The minimum time for hardening is generally several hours at room temperature. Also the use of adhesives requires more handling by personnel and is more fault sensitive. Also the use of adhesives adds weight to the panel and decrease the weight-to-strength ratio.
Another drawback of the prior art is the protruding head disc and bottom flange of the insert when secured into the sandwich panel.
One of the objects of this invention is to provide a method for securing a thermoplastic insert, which is placed inside a through hole in a sandwich panel, to vertical hole walls of the thermoplastic skins of said sandwich panel. The insert for use with the new securing method comprises an essentially cylindrical body with a cylindrical through bore to receive a fastener. It is another object of the invention that ultrasonic energy be applied in order to secure the insert to the thermoplastic skins of the sandwich panel. Another object of the invention is to provide the thermoplastic insert with a concentric thermoplastic rim at the top and bottom of the body, which acts as sacrificial thermoplastic matter and is fused together with the thermoplastic material on top and bottom skins of the sandwich panel as a result of the appliance of ultrasonic energy.
It is an object of this invention to provide a method for securing a thermoplastic insert to thermoplastic skins of a sandwich panel which takes not more than 10 seconds in time and results in a watertight connection. The connection between thermoplastic insert and thermoplastic skins of the panel is made by fusing thermoplastic sacrificial matter of the insert with the thermoplastic skin by applying ultrasonic energy.